Shinichi Baba

Nanoindentation behavior of a two-dimensional carbon-carbon composite for nuclear applications

Abstract Nanoindentation tests on a carbon fiber and carbon matrix composite (C/C composite) for nuclear applications were carried out over a load range from 50 μN to 20 mN on two different crosssections, normal and parallel to the fiber axis. For reference, an isotropic nuclear graphite was also examined. Both the composite and graphite revealed a large amount of elastic recovery on the first loading-unloading indentation curves and inelastic hysteresis on the subsequent cycling curves. The analysis of mean indentation pressure data on the basis of the Weibull statistics indicates that the data on the C/C composite are separated into two groups with different Weibull moduli; the group of the lower modulus with higher mean pressure is attributed to the fiber, and the higher modulus with lower mean pressure to the matrix. The average of mean indentation pressure is obtained as 101 MPa for the fiber and 35.5 MPa for the matrix, while graphite has an intermediate value of 52.2 MPa. There is a slight dependence of mean stress pm on the applied load P in all the materials. However, the trend appears to be different between the two materials; pm decreases with increasing P for the C/C composite, while the reverse is true in graphite. Youngs moduli of the graphite and the two constituents of the C/C composite are estimated from the stiffness of the unloading indentation curves at 10.7 GPa for the graphite as 6.71 GPa for the fiber and 1.97 GPa for the matrix. The values estimated for the graphite and the fiber are comparable to those of the bulk graphite and C/C composite, respectively.

Irradiation effects on thermal expansion of SiC/SiC composite materials

Abstract Irradiation-induced dimensional change and thermal expansion of two kinds of composites, self-particle reinforced SiC p /SiC composites and a Hi-Nicalon™ SiC fiber reinforced SiC f /SiC composite, and monolithic α-SiC were measured after irradiation at 0.2 dpa with irradiation temperatures of 573, 673 and 843 K using the JMTR. From the measurement, swelling was observed for the SiC p /SiC composites and the monolithic α-SiC, on the contrary, the SiC f /SiC composites showed a shrinkage. The measured thermal expansion increased with increasing the specimen temperature below the irradiation temperature, and then rapidly decreased over the irradiation temperature. The so-called ‘temperature monitor effect’ of the silicon carbide was clearly observed for all specimens, the monolithic α-SiC and both composites.

Fundamental thermomechanical properties of SiC-based structural ceramics subjected to high energy particle irradiations

SiC composites have a potential to be used as in-vessel structural components in a future fusion reactor concept. To investigate the fundamental process for deformation and fracture of polycrystalline ceramics, α-SiC monolith mini-specimens were irradiated with heavy ions, 180 MeV Au and 90 MeV Ni. The specimens were tested in bending after different degrees of exposure to elucidate the strength change due to artificial near-surface defects. A microindentation experiment was also carried out to characterize the irradiation damage effects resulting from near-surface defects. Moreover, an FEM analysis was carried out by taking account of the irradiation damage at near-surface region, and the result was compared with the experimental results. This paper presents the particle irradiation effect on the bending strength on the basis of both experimental and analytical results.

Nanoindentation test on electron beam-irradiated boride layer of carbon-carbon composite for plasma facing component of large Tokamak device

The nanoindentation behavior of an electron irradiated boride layer overlaid on carbon-carbon (C/C) composite for plasma facing components of a large Tokamak device was examined up to the maximum load of 0.3 mN. The scanning electron micrography indicated that the irradiation caused the melt-down and resolidification of the boron carbide layer. However, it was found by the X-ray diffraction analysis that the layer did not change its crystalline structure. Youngs modulus and mean pressure of the irradiated area were estimated from load-penetration depth (P-h) curves as 270 GPa and 19.2 GPa, respectively, which were 40 and 20% lower than the values for the bulk material. These properties for the unirradiated region were 125 GPa and 3.1 GPa, respectively, only one third and one tenth of the values for the bulk. It is attributed to the porous structure of the unirradiated material.

High energy neutron and charged particle irradiation effects on thermomechanical properties of carbon–carbon composites for divertor applications

To assess the degradation of carbon-carbon (C/C) composite materials for the divertor structure, 14 MeV neutron and charged particle simulation irradiations were performed on several grades of C/C composites. For the charged particle irradiation the damage to be caused was also estimated using EDEP- 1 code. Materials used were several grades of C/C composites, all of which were used in the JT-60 as divertor armor tiles. The measurement of thermal diffusivity up to 1600 K, electrical resistivity during 14 MeV neutron irradiation and the micro-indentation test at room temperature were performed. The strength and Youngs modulus were evaluated on the basis of the result of micro-indentation test. Main results are: (1) The maximum of the microhardness was found near the maximum projected range of the C/C composite for charged particles. (2) After the irradiation of 10 MeV protons (<0. 1 dpa), 10-90% increases in tensile strength and Youngs modulus were observed, depending on the grades of C/C composites. (3) Softening of C/C composites was observed in the micro-indentation test when they were irradiated by 14 MeV neutrons up to a fluence of 10 19 n/m 2 .